The emission of acidifying gases, such as sulphur dioxide, from e.g. coal-fired power plants has become a major environmental problem. Several different methods have been tried in order to reduce emissions of this type.
To this end, it has been suggested both to purify the fuel and to take measures during the combustion and/or to clean the flue gas produced. Certain pollutants, such as nitrogen oxides, can be dissolved in harmless substances. The formation of pollutants can also be counteracted or obviated by optimizing the combustion process. Other substances, such as sulphur, can however be taken care of only as a residual product and must not be emitted in any form in an uncontrolled manner. In order to remove sulphur, for example, an absorbent is generally added which contains substances forming together with sulphur stable compounds, either in the combustion chamber or in a specially designed flue gas cleaning system.
Examples of prior-art methods are given in SE-8005571-8 involving the injection of absorbent into the combustion chamber, DE-36 07 357 involving wet flue gas cleaning, SE-7904382-4 involving wet-dry flue gas cleaning, EP-0 177 896 involving completely dry flue gas cleaning, and SE-8505269-4 disclosing a combinatory solution.
When supplying absorbent in the combustion chamber, the major drawback is the low degree of utilization of the absorbent. The wet flue gas scrubbers are far more efficient in this respect, but involve high investment costs and maintenance problems.
Wet-dry flue gas cleaning entails lower investment costs but requires a more refined and thus more expensive absorbent than the wet flue gas scrubbers. To avoid precipitation of moisture in the flue gas cleaning system, the absorbent is supplied, in the wet-dry methods, dissolved or suspended in an amount of water which is smaller than the amount required for cooling the flue gas to a temperature below the saturation temperature. The amount of liquid supplied and, hence, the amount of absorbent supplied are thus limited by the heat content of the flue gas.
The completely dry methods involve the lowest investment costs for the flue gas cleaning systems, but most often require a highly refined and, thus, expensive absorbent and do not permit high-grade cleaning. The main reason for this is that solid phase reactions are slow at the temperatures normally prevailing after a coal-fired boiler.
To obtain high-grade cleaning by means of a simple flue gas cleaning system which does not include means for producing, transporting or atomizing an absorbent suspended in water, different techniques intermediate between the dry and wet-dry methods have been suggested.
SE-7908674-0, for example, suggests separate moistening of the flue gases before introduction into a bag filter, the bags of which have been coated with a dust cake of calcium hydroxide. SE-8504675-3 instead suggests moistening of the absorbent with water before it is supplied to the flue gas. The amount of water then must not be larger than as to permit maintaining the powder form of the absorbent.
None of these techniques however yields sufficiently effective flue gas cleaning according to current environmental standards. Therefore, many combinatory solutions have been suggested, according to which a completely dry absorbent as well as an absorbent suspended in water are added. Examples hereof are given in DE-32 35 341 where a dry absorbent is added upstream of an absorbent in slurry form, in SE-8500612-0 where the fresh absorbent is added in dry form while the recycled one is added in the form of a slurry, and in EP-211 458 where an absorbent in dry form and an absorbent in slurry form are added separately or together in the same contact reactor depending on operational conditions.
To supply the absorbent, different types of nozzles have been developed in compliance with the conditions in which they are to operate. Examples of nozzles for supplying and atomizing a slurry having a high dry solids content are given in EP-079 081 and EP-185 630. Examples of nozzles for supplying a completely dry pulverulent absorbent are given in SE-8500612-0. Examples of nozzles for supplying water alone or suspensions having a low dry solids content are given in DE-26 27 880, U.S. Pat. No. 4,625,916 and WO-A-9112084.
DE-26 27 880 describes a nozzle substantially made of two concentric tubes or one tube with a surrounding chamber. Liquid is pumped in the inner tube, and compressed air is supplied to the outer tube or the chamber. The partition is formed with a plurality of holes, through which compressed air flows into the inner tube and mixes with the liquid. The mixture leaves the tube with a step form pressure drop producing an effective disintegration of the liquid which requires little energy.
U.S. Pat. No. 4,625,916 describes a nozzle similar to that just mentioned but which terminates in a divergent channel and, thus, probably does not give rise to any step form pressure drop at the outlet. Furthermore, the partition has holes arranged in a specific pattern in order to make the mixing of air and liquid efficient.
WO-A-9112084 reveals a nozzle for atomizing a liquid to be injected into a process gas, e.g. in the separation of arsenic from a gas containing the same. The nozzle comprises a central mixing space through which the atomizing gas is flowing substantially rectilinearly and into which the liquid is supplied inwards through essentially radially bored holes. Pressurized air and liquid is supplied to the nozzle in two separate parallel tubes. The nozzle comprises several pieces which are threaded together and the leakage, between the inlets and to the surroundings is prevented by gaskets.
These nozzles have been found to be liable to leakage, the liquid escaping into the air duct in places where not desired, and to pressure adjustment difficulties, where inequilibrium means that liquid escapes through the holes which are intended for the intake of air. If the liquid contains dry matter, such malfunctions soon result in deposits of material in the air duct system and time-consuming cleaning operations to restore proper function.